<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<style type="text/css">
    body{
        background-image: url("https://img-blog.csdnimg.cn/e90664acbb424877adfaa40ca21cd35b.jpeg");
    }
    .glass{
        background: inherit;
        width: 400px;
        height: 400px;
        margin-left: 500px;
        position: absolute;
        overflow: hidden;  //adding overflow hidden
    }
    .glass:before{
        content: "";
        width: 400px;
        height: 400px;
        background: inherit;
        position: absolute;
        left: -25px;  //giving minus -25px left position
    right: 0;
        top: -25px;   //giving minus -25px top position
    bottom: 0;
        box-shadow: inset 0 0 0 200px rgba(255,255,255,0.2);
        filter: blur(1000px);
    }
</style>

<body>
<div class="glass" >

</div>
</body>
</html>